1. Field of the Invention
The present invention relates to an improvement of a piezoelectric actuator in which a piezoelectric element and a lead wire for supplying a drive signal to the piezoelectric element are joined.
2. Description of the Related Art
Recently, a piezoelectric actuator which obtains a drive force by utilizing a piezoelectric effect of a piezoelectric element has been attracting attention in the field of microactuators.
The piezoelectric actuator heretofore comprises, as shown in FIG. 7, a support 71 having a central hole 71a and a conductive wire through hole 71b, a support shaft 72 disposed in the support 71, a resilient body 74 fixed to the support shaft 72 and having a protrusion 74a, a piezoelectric element 75 joined to a surface of the resilient body 74 in one direction, and a lead wire 77 for supplying an excitation signal, with one end thereof being connected to the piezoelectric element 75. Here, the piezoelectric element 75 has an electrode pattern 76 formed in a thin film and is joined to the resilient body 74 to constitute an oscillating body 73. The lead wire 77 has a base material 77c and conductive wires 77a, 77b disposed on the base material. Moreover, the lead wire 77 is joined thereto by plating a tin-lead solder thereon to form a junction layer 78 beforehand, pressing the lead wire 77 and the piezoelectric element 75 to bring them into contact with each other and applying heat.
However, it is necessary that the above-described lead wire 77 be firmly joined in the junction layer 78 with the piezoelectric element 75 to secure an adhesion strength, thereby causing a problem in that if the joining rear is increased, it interferes with oscillation of the oscillating body 73. This is due to the principle that the oscillation of the oscillating body 73 caused at the time of driving the piezoelectric actuator is smallest in the vicinity of the support shaft 72, and as the joining rear expands in the peripheral direction, or the joined position shifts outward, even a lead wire 77 having a low rigidity interferes with the oscillation.
Moreover, in the joining step, since a solder material flows out along the lead wire at the time of heating the solder to increase the joining area, it is necessary to set the heating conditions, taking the outflow of the solder into consideration. However, since it is difficult to control the outflow volume of the solder, there is a problem in that poor joining strength due to a small joining area and poor oscillation characteristic due to a large joining area are caused, to thereby decrease the yield rate. Furthermore, variations in the joining positions of the lead wire 77 causes a problem such as a poor oscillation characteristic and decrease in the yield rate.
It is, therefore, an object of the present invention to provide a piezoelectric actuator which reduces variations in the joining areas and the joining positions of a lead wire and a piezoelectric element, and maintains the oscillation characteristic, as well as improving the yield rate by securing a proper joining strength in the joining step of the piezoelectric element and the lead wire.
Specifically, in order to solve the above problems, a first aspect of the present invention is a piezoelectric actuator obtained by joining a piezoelectric element having an electrode formed thereon and a lead wire for supplying a drive signal to the piezoelectric element, having a melt material, at the junction portion of the electrode of the piezoelectric element and the lead wire, in a substantially spherical shape for being welded on respective materials.
Here, the lead wire is constituted of a conductive wire, or by firmly fixing a conductive wire and a nonconductive base material, and the conductive wire includes aluminum, copper, silver, gold or an alloy of these. The nonconductive base material includes, for example, polyimide resin, silicon resin, polyethylene resin, and epoxy resin mixed with glass fibers. The conductive wire and the nonconductive base material can be produced by adhesion, electroforming, printing or the like. Moreover, the melt material can be produced by processing a tin-lead solder having a different composition ratio of, for example, 6:4 or 9:1, tin alloy, tin, copper, aluminum, nickel or the like into a substantially spherical body, a cubic body, a cylindrical body or the like.
In this way, the melt material which is arranged in the junction portion of the piezoelectric element and the lead wire becomes a substantially spherical body due to heating at the time of joining, and becomes a spherical body collapsed in the pressing direction, that is in the thickness direction due to the application of pressure at the time of joining. If the diameter of the melt material extending along the piezoelectric element and the lead wire and the width of the lead wire are multiplied, the joining area can be easily calculated. FIG. 6 is a graph showing the increase quantity of the diameter when the substantially spherical melt material is collapsed due to the application of pressure at the time of joining. The X-axis denotes a collapsed quantity in the thickness direction of the melt material, assuming a diameter of the melt material before application of pressure to be 1, and the Y-axis denotes an increase quantity thereof expanding along the piezoelectric element and the lead wire. Here, if a substantially spherical melt material is used, as shown in FIG. 6, the collapsed quantity in the thickness direction changes depending upon the pressurizing conditions at the time of joining. For example, if the melt material is collapsed up to {fraction (1/10)} of the diameter (X-axis: 0.1), the junction length can be calculated as about five times the diameter (Y-axis). Therefore, control of the joining area becomes easy by processing the size of the melt material beforehand so as not to suppress the oscillation characteristic. Moreover, in the step of joining the piezoelectric element and the lead wire, a proper junction strength can be secured by measuring the diameter of the melt material after being joined, and reflecting the measurement in the joining conditions such as welding pressure or the like.
Furthermore, a second aspect of the present invention is a production method of a piezoelectric actuator obtained by joining a piezoelectric element having an electrode formed thereon and a lead wire for supplying a drive signal to the piezoelectric element, characterized by having steps of: supplying a melt material to the lead wire; welding the melt material on the lead wire; and welding the melt material on the piezoelectric element.
Here, the melt material can be obtained by processing a tin-lead solder having a different composition ratio of, for example, 6:4 or 9:1, tin alloy, tin, copper, aluminum, nickel or the like into a desired shape.
As a result, positioning of the melt material at a proper position on the lead wire becomes possible, by welding the lead wire and the melt material, immediately after the melt material is supplied to the lead wire. Therefore, variations in the joining position of the piezoelectric element and the lead wire are reduced, thereby improving the yield rate.
Moreover, the production method is characterized by using a melt material having a substantially spherical shape as the melt material.
As a result, in the step of welding the substantially spherical melt material on the lead wire and the piezoelectric element, the melt material joins the lead wire and the piezoelectric element, while maintaining the substantially spherical shape. That is to say, the melt material does not flow out along the lead wire, and the joining area is stable, thereby improving the yield rate.
Furthermore, in the step of supplying the melt material to the lead wire, there is also a step of immersing the melt material in a fusing agent and supplying the melt material to a transfer plate, a step of transferring the melt material from the transfer plate to a transfer tool, and a step of transferring the melt material from the transfer tool to the lead wire.
Here, the fusing agent contains an acid or alkaline aqueous solution, or an organic solvent.
As a result, it becomes possible to supply a minute amount of melt material to the lead wire. Also, by supplying a proper number of melt materials to the transfer plate and then transferring these to the lead wire using the transfer tool, the quantity of melt material supplied to the lead wire can be controlled.
Moreover, a production method of a piezoelectric actuator obtained by joining a piezoelectric element having an electrode formed thereon and a lead wire for supplying a drive signal to the piezoelectric element is characterized by having steps of: immersing a melt material in a fusing agent and supplying the melt material and the fusing agent to a transfer plate; transferring the melt material and the fusing agent from the transfer plate to a transfer tool; supplying the melt material and the fusing agent from the transfer tool to the lead wire; and welding the melt material on the lead wire and the piezoelectric element.
In this way, minute melt material can be easily supplied to the lead wire, and the melt material can be formed in a substantially spherical shape by heat for joining the lead wire and the piezoelectric element.